1. Field of the Invention
The present invention generally relates to the fabrication of a semiconductor device, and particularly, to a so-called pre-cleaning process for removing native oxide formed on the surface of a silicon substrate.
2. Description of the Related Art
Native oxide formed on the surface of a silicon substrate needs to be removed before various processes such as an insulation film forming process and an epitaxial process on the silicon substrate. The native oxide includes, for example, an oxide film formed due to the atmospheric oxidation of hydrophobic silicon surface formed by cleaning the silicon surface with diluted fluorine acid and an oxide film unintentionally formed in oxidation processing, dilution processing, and CVD processing, for example, even at a low temperature.
The native oxide needs to be completely removed in the recent fabrication of super fine semiconductor devices in which, for example, a high dielectric gate insulation film, which is 1 nm thick or less, needs to be formed. The native oxide also needs to be removed not only for forming a super thin insulation film such as the high dielectric gate insulation film but also for forming a silicon/metal contact and metal silicide.
The native oxide is removed from the surface of a silicon substrate in dry atmosphere so that the silicon substrate the native oxide of which is removed is transported in vacuum to the next step with its cleaned surface kept as clean as it is. Thus, the removal of the native oxide is referred to as dry cleaning.
For example, the following methods for dry cleaning processing are known: a method of processing the surface of the silicon substrate in anhydrous HF gas, a method of processing the surface of the silicon substrate in an atmosphere of HF gas and water vapor, and a method of processing the surface of the silicon substrate in an atmosphere of F2 gas excited by ultra violet ray thereby to generate fluorine radicals with additional H2 gas.
Japanese Laid-open Patent Application No. 7-321046 is known as a prior art literature related to the present invention.
The method of processing the surface of the silicon substrate with anhydrous HF gas is a method in which the native oxide is removed by reacting remnant moisture contained on the surface of the native oxide or in the native oxide with the anhydrous HF gas thereby to form HF aqueous solution. The native oxide can be selectively removed by controlling the density of HF.
The method of processing the surface of the silicon substrate in an atmosphere of HF gas and water vapor is a method in which HF gas and H2O gas condense on the native oxide into HF aqueous solution thereby to etch the native oxide. When the etching of the native oxide is completed, the supply of the HF gas and the water vapor is terminated. The liquid condensed on the silicon substrate evaporates.
One of the problems of the conventional methods is that a large amount of fluorine (F) atoms remain on the surface of the processed silicon substrate after processing, and residual substance is easily left after the liquid evaporates. Another problem of the conventional methods is caused by organic substance absorbed on the surface of the silicon substrate that is difficult to remove before the removal of native oxide. The organic substance may cause the residual substance.
In the case of the method of processing a substrate surface with fluorine radicals generated by exciting an atmosphere of F2 gas and hydrogen gas with ultra violet light, the problem that residual substance remains on the substrate surface after processing does not occur. However, a large amount of fluorine (F) atoms are absorbed on the surface of the silicon substrate. The fluorine atoms need to be removed after processing for removing native oxide.